There is known a sensor apparatus which measures the properties or constituents of an analyte liquid by detecting a object to be detected object to be detected contained in the analyte liquid with use of a detecting element such as a surface acoustic wave device (refer to Patent Literatures 1 to 3, for example).
For example, in a sensor apparatus employing a surface acoustic wave device, a reaction section which undergoes reaction with a component contained in a sample of an analyte liquid, is disposed on a piezoelectric substrate, and the properties or constituents of the analyte liquid are detected by measuring variation in a surface acoustic wave propagating through the reaction section. Such a measurement method using the surface acoustic wave device or the like has the advantage over other measurement methods (for example, enzymatic method) in that it lends itself to simultaneous detection of a plurality of characteristics to be measured.
However, in such a conventional sensor apparatus, on the upper surface of the piezoelectric substrate, a pair of IDT electrodes and a region of the reaction section located between the paired IDT electrodes are equal in level, wherefore sufficient concentration of surface-acoustic-wave energy on the reaction section cannot be achieved with ease, which results in difficulties in detecting an object to be detected contained in an analyte with high sensitivity.